Thymosin fraction 5, known for example from U.S. Pat. No. 4,082,737, is a potent immunopotentiating preparation and can act in lieu of the thymus gland to reconstitute immune functions in thymic deprived and/or immunodeprived individuals. Ongoing clinical trials with fraction 5 suggest that thymosin is effective in increasing T cell numbers and normalizing immune function in children with thymic dependent primary immunodeficiency disease and can increase T cell numbers in immunodepressed cancer patients.
The first active peptide isolated and characterized from thymosin fraction 5 has been termed thymosin alpha.sub.1. See for example U.S. Pat. No. 4,079,127 for a description of this peptide's isolation and characterization. Synthesis of alpha.sub.1 by solution and solid phase synthesis techniques is described in U.S. Pat. No. 4,148,788. Additionally the synthesis of thymosin alpha.sub.1 by solution phase procedures is shown in U.S. Pat. No. 4,116,951. Thymosin alpha.sub.1 has been found to be one or more orders of magnitude more active than fraction 5 in several in vitro and in vivo assay systems designed to measure T cell differentiation and function. Thymosin alpha 1 is currently in the clinic to determine its efficiacy in the treatment of immunodeficiency diseases, immunodepressed cancer patients and in the prevention of opportunistic infections in immunosuppressed patients.
Thymosin alpha.sub.11 shares the biological activities of thymosin alpha 1 and in view of its structure relationship to thymosin alpha.sub.1 appears to represent a proteolytically modified fragment of the precursor native thymic peptide. However, since it is a large fragment and thus closer in structure to the native peptide it would represent a clinically preferred therapeutic substance.